1. Field of the Invention
This invention generally relates to telephony networks and, more specifically, to a telephony switching system that incorporates concentrators for multiplexing calls from a large number of telephone subscriber lines onto a substantially reduced number of highways for transfer to a central office switching network.
2. Description of the Prior Art
U.S. patent application Ser. No. 507,935, filed June 23, 1983, discloses a telephony network that includes a digital central office for performing switching operations. In this network, telephone lines from subscribers and trunk circuits from other central offices connect directly to a digital central office through a plurality of line and trunk circuits in port group units. Each connection is made through conventional tip and ring, or similar, conductors that extend from each individual subscriber or remote central office to the location of the digital central office. These conductors carry signals in analog form that represent voice signals, data, or other user information, herein generally referred to as voice information, and supervisory information. The supervisory information may be "sense supervisory" information, including hook status and dial pulse information received from the subscriber, or "control supervisory" information, including ringing and other control signals that are sent over the subscriber line or that cause certain operations to be performed on the subscriber telephone.
Each port group unit in the central office described in U.S. patent application Ser. No. 507,935 connects directly to a plurality of telephone subscriber lines through individual port circuits, such as line or trunk circuits. Each port circuit converts incoming analog voice signals to digital form that are transferred in a serial pulse train and multiplexed onto a port group highway with signals from other subscriber lines. Sense supervisory information, in digital form, is also multiplexed onto this pulse train. A time slot interchange (TSI) matrix network receives the pulse train and strips the incoming sense supervisory information for storage in an area of a port data store that is assigned to each port circuit. A port event processor samples the information in each port data store area, processes the information and uses it to send messages to a call control processor. The call control processor sends information, including commands, to the individual areas of the port data store to enable the port event processor to control the corresponding telephone subscriber line and to the TSI matrix network for establishing a switching channel through the network to establish a path for the digitized voice signals to the port serving a called subscriber line.
Commands to the port event processor from the call control processor enable the transmission of a dial tone, termination of a dial tone, or ringing signals to be applied to the calling and called subscriber lines. The port event processor generates control supervisory information in response to these commands. The control supervisory information is multiplexed with the voice information in digital form for transmission to the port group unit connected to the port group highway. Then the corresponding port group unit performs various functions in response to the commands and converts the digital voice data signals into analog form for transmission through a particular port circuit to the subscriber's telephone lines.
Digital central offices of this type require individual telephone lines from each subscriber location to the location of the digital central office. This approach works well and is economical where the subscribers either are located in a relatively small geographical area around the central office, or are randomly, but widely, dispersed geographically centered around the digital office. However, it often occurs that telephone subscribers are located in clusters geographically remote from the central office. For example, subscribers may live in small towns, in a rural setting, or in apartment houses, in an urban setting.
In such situations, in the system of application Ser. No. 507,935, a line must be strung from the central office to each remote subscriber's location. This line may include not only cabling, but also gain devices such as amplifiers, for each subscriber line. The expense of the additional cabling is readily apparent, especially in view of the fact that in many such situations the traffic volume in such remote networks, as a percentage of maximum traffic capacity, is very low. Thus, the actual utilization of the telephone lines can become quite expensive and inefficient.
One approach as disclosed in U.S. Pat. No. 4,393,495, is to place remote port units at the center of subscriber clusters and establish a reduced number of communications links between the clusters and the central office. One problem with such an arrangement is that, as traffic increases, the number of lines between the remote port unit and the central office remains fixed. Therefore, as traffic increases, additional remote port units must be established to carry the additional traffic.
The arrangement of U.S. Pat. No. 4,393,495 allows signals from a specified group of telephone subscriber lines to be multiplexed onto a particular span to the central office. When the span's traffic carrying capacity is full, no more calls can be accepted from that group of subscriber lines, even if additional spans may be available to that same geographical area of subscribers through other remote ports. The additional spans are connected only to other subscriber lines through the other ports, which substantially reduces the traffic-carrying flexibility of the system.